Antoine Doinel is a young boy growing up in Paris. Misunderstood by his parents for playing truant from school and stealing and tormented in school for discipline problems by his teacher (such as writing on the classroom wall, and later falsely explaining his absence as having been due to his mother's death), Antoine frequently runs away from both places. He finally quits school after his teacher accuses him of plagiarizing Balzac. (Antoine loves Balzac and in a school essay he describes "the death of my grandfather", in a close paraphrase of Balzac from memory.) He steals a Royal typewriter from his stepfather's workplace to finance his plans to leave home, but, having been unable to sell it, is apprehended while trying to return it.
The stepfather turns Antoine over to the police and Antoine spends the night in jail, sharing a cell with prostitutes and thieves. During an interview with the judge, Antoine's mother confesses that her husband is not Antoine's biological father. Antoine is placed in an observation center for troubled youths near the seashore (as his mother wished). A psychologist at the center probes reasons for Antoine's unhappiness, which the youth reveals in a fragmented series of monologues.

While playing football with the other boys one day, Antoine escapes under a fence and runs away to the ocean, which he has always wanted to see. He reaches the shoreline of the sea and runs into it. The film concludes with a freeze-frame of Antoine, which, via an optical effect, zooms in on his face as he looks into the camera.